Jimmie D wrote:

Interesting, when I replaced the mica insulator on a screen bypass cap with
Teflon the capacitance increased. I guess this is because the Teflon was
considerably thinner than the mica.

That would be the reason. The dielectric constant of mica is around 5-7
depending on the type. So for the same thickness, mica would give you at
least 2-1/2 times the capacitance of Teflon.

Dielectric constants for a wide range of materials are readily available
on the web. Google is your friend.

Roy Lewallen, W7EL